HOW TO: Choose and/or Adjust your Blow Off Valve

It’s a common misconception that all blow off valves are the same in performance and function. Sure, some make different sounds than others, and have different internal materials that may last longer, but it actually goes deeper than that.

Note – This article applies to both blow off valves, and recirculating valves.

First off, let us discuss “turbo sneeze”. This is what happens when your blow off valve is too stiff or restrictive, or if you don’t have one at all. This is kind of weird, but this is what it sounds like:

It can also be in the form of chatter or “flutter.” When searching for these videos, I got the impression that for some reason people think this is cool and/or good.

Make no mistake, this is not good. Your turbo is going from a bajillion (yes, a bajillion) RPM down to almost nothing in a very short period of time. It doesn’t like it, and it doesn’t like you if you do it to it.

Blow off valves are actually quite similar to wastegates. There’s some diaphragms, some springs, and some valving. The diaphragm is designed to move when there is a pressure differential between your charge piping and your manifold (basically, when you close your throttle and your engine goes to vacuum but your turbo is still trying to push air). This pressure differential causes the blow off valve to open, and you give the pressurized air a place to go before you get a really big spike and stall your turbo.

Note – keeping your turbo spinning between shifts also reduces lag. If you stall your turbo, you have to get that joker back up to speed, which takes time.

So, let’s get down to the nitty-gritty. It all comes down to how easy the valve is to open (how much pressure differential there has to be before it moves), how quickly it opens, and how much it opens (which, among a few other things, determines how much it can flow).

At this point we look to our dear friend Sir Isaac Newton: F = MA This comes down to the spring(s) you have in there – the pre-load they have & the spring rate, the mass of the piston, and the size of the diaphragm(s). Unfortunately you, as the consumer, aren’t exactly privy to a lot of this information. You CAN control the size of the ports going into the BOV (from the charge pipe – bigger is better), the length of that port (shorter is better) and signal line going to the valve from the manifold (bigger is better… this isn’t just a “signal” line, you’re actually moving diaphragms here, so you need some volume). Some blow off valves are adjustable. You’ll want to back off on the pre-load (go counter-clockwise) as much as you can if you’re getting sneeze/flutter.

If you feel you’ve optimized these things (or they are out of your control for whatever reason), and you’re still having sneezing or flutter problems, its time to crack the valve open and try replacing some springs.

We’ll demonstrate using a Greddy Type S blow off valve. Let me start by saying I don’t like it. More on that later. Here it is, removed from the vehicle:

Yeah, its missing a bolt. Whatevs. Remove the bolts except for 2 that are opposite of each other. Contents are under spring load, so put some pressure on the cap while you remove the last 2 bolts (evenly). The top should pop off and reveal the guts. This unit has 2 springs. Internet wisdom suggests removing the small spring gets this BOV to acceptable performance levels for most automobiles. I have yet to read a review that suggests this valve worked for anyone right out of the box. Seems like literally everyone has to do this spring mod. Here is a handy chart that the DSM guys made that shows basically everything we’ve been talking about until now:

This valve also has an integrated secondary boost source – the bottom nipple that seems to confuse the heck out of everyone, as is evident by this picture here: I’m not sure why they don’t recommend connecting it, but in my eyes, you want all the assist from the boost pressure as you can get, so I leave it connected. Of course, to connect it, you have to drill a damn hole in your intercooler piping and put a fitting on it… not sure why they don’t just incorporate it into the valve… Anywho, once you’ve decided what setup you’d like to use, you re-assemble the valve. Put the little top hat doohickey on there, make sure the adjustment screw is backed all the way out, and put the valve cap back on (again, EVENLY!). Make sure the diaphragm is seated normally. Adjust your adjuster down to where you think it needs to go (take a guess if you have to), put it back on your charge piping, hook the lines back up, and take it for a ride.

Note, I still get VERY slight turbo sneeze. I’ll have to try backing off on the adjuster a bit. If that doesn’t work, I’ll have to swap in the small spring. Unfortunately, this valve now opens while I’m driving and makes a wicked annoying whistle as I go down the road at 13-10 inches of mercury. If the turbo was bigger, it wouldn’t have this problem (wouldn’t be making as much pressure at that amount of vacuum & RPM).

One more note: Venting to atmosphere? Silly. Save that for the kids and their poser cars revving in the parking lot at the mall, trying to impress high school girls who might have seen F&F 7. Plumb that sucker in (recirculate) for easier tuning, increased performance, and a more professional image.

Thanks for joining us for “BOV for Dummies” – We hope you found this article useful. Happy boosting!